Generally, a liquid crystal display (LCD) controls the amount of light transmitted from liquid crystal cells in response to video signals to thereby display a picture on a liquid crystal panel. The cells are typically arranged in a matrix pattern. The liquid crystal panel includes liquid crystal cells arranged in an active matrix type and driving integrated circuits (IC's) for driving the liquid crystal cells.
The driving ICs are usually manufactured in chip form and mounted on a tape carrier package (TCP) film attached to the outer periphery of the liquid crystal panel. The ICs are also connected by a tape automated bonding (TAB) system mounted along the outer periphery of the liquid crystal panel when the IC's are connected by a chips-on-glass (COG) system.
In the case of TAB system, the driving IC's are electrically connected to a pad portion disposed along an edge of the liquid crystal panel by the TCP. The pad portion is connected to electrode lines, which are in turn connected to each liquid crystal cell of the liquid crystal panel, to apply driving signals generated from the driving IC's to each liquid crystal cell.
FIG. 1 is a plan view showing a structure of a conventional liquid crystal display panel. As shown, the liquid crystal panel 2 includes a lower plate 4 and an upper plate 6 bonded to each other. The liquid crystal panel 2 also includes a picture display part 8 having liquid crystal cells arranged in a matrix pattern; gate pads 12 and data pads 14 connected between driving IC's (not shown) and the picture display part 8; gate links 34 and data links 16 for connecting the gate pads 12 and the data pads 14 to the picture display part 8, respectively; and a seal 10 provided at the outer periphery of the picture display part 8 so as to bond the lower plate 4 to the upper plate 6.
Within the picture display part 8, a plurality of data lines 13 intersect with the plurality of gate lines 11 on the lower plate 4. A video signal is applied to each data line 13 via the data pad 14 and the data link 16 and a scanning signal is applied to each gate line 11 via the gate pad 12 and the gate link 34. At each intersection, each liquid crystal cell is provided with a thin film transistor (TFT) and a pixel electrode connected to the thin film transistor. The TFT provides a switching function to apply a data signal to drive the liquid crystal cell.
Red, green, and blue color filters are formed on the upper plate 6. The color filters are separated by a black matrix and a common transparent electrode is formed on the surfaces of the color filters.
The lower plate 4 and the upper plate 6 are spaced apart by a spacer to provide a constant cell gap. The lower plate 4 is bonded to the upper plate 6 by the seal 10, which is positioned along outer edges of the picture display part 8. The cell gap area is injected with liquid crystal to form the liquid crystal layer, and thereafter is sealed.
The gate pads 12 and the data pads 14 are located at the edge of the lower plate 4 not overlapped by the upper plate 6. Each gate pad 12 applies a scanning signal from the gate driving IC to the gate line 11 via a wire within the TCP film and the gate link 34. Also, each data pad 14 applies a video data signal from the data driving IC to the data line 13 via the data link 16.
In the conventional liquid crystal panel 2 as described above, a protective film is coated on the entire lower plate 4 to protect the metal electrode lines and the thin film transistors. Also the pixel electrode, which is connected via a contact hole to the TFT, is formed on the protective film for each cell area. The pixel electrode is a transparent electrode made from indium tin oxide (ITO), which has a relatively strong durability.
Generally, an inorganic material such as SiNx or SiOx is used as the protective film. These typically have high dielectric constants. Because of the high dielectric constants, the conventional liquid crystal with inorganic protective films suffers from a coupling effect caused by an increase in parasitic capacitance between the pixel electrode and the data line 13.
A way to minimize the coupling effect is to keep the two electrodes at a relatively long distance, for example, of 3 to 5 μm so that the pixel electrode dose not overlap with the data line 13. However, due to the increased spacing, it is necessary to form an area of the pixel electrode applying a voltage to the liquid crystal layer to be as narrow as possible. In such instance, aperture ratio of the liquid crystal cell, which depends on the area of the pixel electrode, is reduced.
A way to overcome this problem, i.e. minimize the coupling effect but still achieve higher aperture ratio, is to use protective films made of organic materials. Organic materials such as benzocyclobutene (BCB), spin on glass (SOG), or Acryl, have relatively low dielectric constants. Due to the low dielectric constants, the area of the pixel electrode can be enlarged to improve aperture ratios of the liquid crystal cell.
Unfortunately, a high aperture ratio LCD employing the organic protective film suffers from problems of its own. When bonding the upper and lower plates, a seal is used. As shown in FIG. 1, the seal 10 makes contact with the organic protective film (shown in FIGS. 3A and 3B) as the plates are bonded.
Typically, epoxy resin is used as the seal. Such seal strongly adheres to inorganic protective films and glass substrates, but weakly adheres to organic materials such as the organic protective film. Thus, the high aperture ratio LCD employing the organic protective film is much more likely to develop leakage problems when the liquid crystal panel is subjected to physical stresses such as an impact.
In addition, the conventional LCD typically has a gate insulating layer disposed between the glass substrate and the organic protective film. Unfortunately, an organic protective film has poor adherence to the gate insulating film as well. Accordingly, a crack may be generated between the organic protective film and the gate insulating film due to physical stresses. As a result, the organic protective film could be floating or the liquid crystal may leak. Such problems of the conventional liquid crystal are described in further detail with reference to the accompanying drawings.
FIG. 2 is an enlarged plan view showing a crossing portion between the data link and the seal in FIG. 1. As shown, the data link 16 is formed along with the data pad 14 and the data line 13. A semiconductor layer 18 extends from the data line 13 into the data pad 14 at the lower portion of the data link 16. The seal 10 is located on the organic protective film in a direction crossing the data link 16. The data pad 14 contacts a transparent electrode 17 on the organic protective film through a contact hole 19 defined in the organic protective film. The transparent film 17 is connected to the data driver IC mounted on the TCP film. The transparent film 17 is designed to protect a metal electrode as well as to prevent oxidation of the metal electrode during the TAB process.
FIG. 3A shows a vertical section of the liquid crystal display panel taken along the 3A–3A′ line in FIG. 2, and FIG. 3B shows a vertical section of the liquid crystal display panel taken along the 3B–3B′ line in FIG. 2. In FIGS. 3A and 3B, the lower plate 4 includes a lower glass substrate 20, a gate insulating layer 22, a semiconductor layers 18, the data links 16, and an organic protective film 24. As shown, the gate insulating layer 22, the semiconductor layers 18 and the data links 16 are sequentially deposited on the glass substrate 20, and the organic protective film 24 covers the entire resulting surface.
The upper plate 6 includes of an upper glass substrate 30, color filters (not shown), a black matrix 28, and a common transparent electrode 26. As shown, the color filters and the black matrix 28 are formed on the upper glass substrate 30, and the common transparent electrode 26 is formed thereon.
The seal 10 bonds the lower plate 4 and the upper plate 6 to each other. As described previously, the seal 10 weakly adheres to the organic protective film 24. In addition, the organic protective film 24 weakly adheres to the gate insulating film 22 due to the inorganic nature of the gate insulating film 22. As a result, the organic floating film 24 may float or crack due to physical stresses thus causing liquid crystal 32 to leak.
FIG. 4 is an enlarged plan view showing a crossing portion between the gate link and the seal in FIG. 1. As shown, the gate link 34 is formed with the gate pad 12 and the gate line 11. The gate pads 12 contacts the transparent electrodes 17 through the contact hole 19 formed in the gate insulating film and the organic protective film. The seal 10 crosses the gate link 34.
FIG. 5A shows a vertical section of the liquid crystal display panel taken along the 5A–5A′ line in FIG. 4, and FIG. 5B shows a vertical section of the liquid crystal display panel taken along the 5B–5B′ line in FIG. 2. In FIGS. 5A and 5B, the upper plate 6 is much like the structure as shown in FIGS. 3A and 3B, respectively. The lower plate 4 is slightly different in that instead of having semiconductor layer and data link disposed between the organic protective film 24 and the gate insulating layer 22, gate link 34 is disposed between the gate insulating layer 22 and the glass substrate 20 (compare FIGS. 3A and 5A).
Again because the organic protective film 24 has weak adherence to both the seal 10 and the gate insulating layer 22, leakage may develop due to physical stresses.